The Sprint Planning is facilitated by the team's Scrum Master and consists of the Scrum Team: Development Team, Product Owner (PO) and Scrum Master (SM). It aims to plan a subset of Product Backlog items into a Sprint Backlog. The Scrum Sprint is normally started in after the Sprint Planning meeting.
In the 'What' phase the team starts with the top of the ordered Product Backlog. The team at least implicitly estimates the items by forecasting what they could take into Sprint Backlog. If needed they could ask / discuss items with the PO, who has to be in place for this meeting.
In the 'How' phase the team shortly discusses every picked Sprint Backlog item with the focus on how they will takle it. SM helps the team not to deep dive into discussion and implementation details. It is very likely and good if more questions to the PO are asked or refinements to the items, or backlog is done by the team.
The team should come up with a shared Sprint Goal for the Sprint to keep the focus in the Sprint time box. At the end of the Sprint Planning, the team forecasts that they can achieve the Sprint Goal and likely complete all Sprint Backlog items. The SM should ensure that the team does not overestimate their targets. They can do so by providing useful insights or statistics.
